This invention relates to methods for the nondestructive eddy-current testing of formed articles such as structural members to determine the presence of hidden faults capable of causing the article to fail in conditions of intended use, and particularly to techniques for automatically locating and characterizing faults in a variety of physical members, including those composed of graphite-fiber composite materials.
While the invention is particularly useful for the nondestructive testing of graphite-fiber composite materials, the method offers advantages which can be used for a large variety of other materials.
Modern graphite fibers came about during a search for a filament to replace glass fibers as a stiffener in thermoset plastics. Glass fibers possess a number of important qualities including low density and great strength. One quality they lack, however, is stiffness. Graphite fibers have this and a number of other important qualities, but required that a practical method be found to produce them. In the early 1960s, experiments were carried out to thermally convert precursor materials into carbon fibers and fabrics. Within five years, carbon and graphite cloth were commercially available and were used extensively in phenolic composites for missile rocket motors. After five more years of development, commercial production of continuous fibers from rayon precursor yielded strong and uniform material. Today, rayon precursors have been almost entirely replaced by polyacrylonitrile (PAN) and pitch precursors.
Graphite fibers (in unidirectional arrays, woven cloths, and chopped fibers) are currently used with a variety of matrix materials. Epoxy (a thermoset plastic) is the most common matrix material, and graphite fibers are increasingly used with thermoplastics. In addition to epoxy and thermoplastics, carbon is sometimes used as the matrix material, yielding a material which can withstand temperatures in excess of 5000 degrees F.
Graphite composite materials offer superior strength and stiffness, dimensional stability, toughness, low weight, relatively low cost, and a variety of production methods. The outstanding design properties of graphite fibers in resin matrices are their high strength- and stiffness-to-weight ratios, and fatigue resistance. Graphite composites are, however, relatively brittle, have no yield behavior, and their resistance to impact is low. Graphite composite longitudinal strength ranges from 110,000 to over 450,000 psi. Longitudinal tensile modulus is in the range of 20-70 million psi. With proper selection and orientation of fibers, composites can be stronger and stiffer than equivalent thickness steel parts, and weigh 40 to 70% less.
Graphite composite materials today are widely used, with common applications including aircraft wings and wing structures for commercial and military aircraft, fuselage panels for military aircraft, cargo doors for space shuttles, light-weight components for racing bicycles, eye-glass frames, tennis racquets, skate-board decks, golf club shafts, skis, and structural components in high-performance racing sculls. Graphite in a carbon matrix is used for rocket nozzles and nose-cones, where performance is paramount and price is not an important consideration.
Dramatic increase in strength and stiffness is achieved by combining graphite-fiber reinforcements with tough matrix resins. Because these composites are frequently designed for structural optimization, their properties and performance can be critical, especially where failure can be life-threatening. Unlike the structure of most other materials, the built-up nature of laminates makes defects more likely to occur. Damage tends to be more upsetting to the balance of factors that affect material performance.
Techniques currently used most often to perform nondestructive testing on graphite composites include visual inspection, the tap test, radiography, and ultrasound. Visual inspection is limited to surface faults. Even impact damage is often sub-surface or visible only on the back side. The tap test provides a response which varies with material and structure, requires expertise, is highly subjective, and provides no quantitative information. Low energy radiography can pinpoint localized variations in fiber density in relatively thin components. Except in cases where longitudinal exposures are appropriate, the injection of radio-opaque material is required for the detection of delamination. In order to do this, the defect must already have been detected by other means and must be open to one surface.
Ultrasound testing is currently the most commonly used procedure for testing graphite composites non-destructively, and is used extensively by end users and manufacturers of graphite composite components alike. Ultrasonic nondestructive testing has the advantages that it is particularly well-suited to locating delaminations in graphite composites, that it can be relatively inexpensive to put in place, and that it is relatively easy to use and interpret. It does, however, have its limitations. First, while ultrasound responds to even the slightest material delaminations, it is insensitive to fiber breakage. Delaminations by themselves do not always indicate that a component is not serviceable. Secondly, it is messy. Immersion tanks or other couplants must be used, which means that either a component must be removed from service to be immersed in a tank, or that it be covered with a couplant before testing. In either case, the tested area will be in poor shape to receive a patch if a problem area is found after testing.
Since graphite fibers are relatively good electrical conductors, nondestructive eddy-current testing is another suitable technique for testing graphite composites. High-frequency magnetic fields cause eddy currents to flow in conductive material. In graphite composites, eddy currents flow in the graphite fibers, and are impeded only if fibers are broken. Eddy current nondestructive testing, therefore, responds to the feature which gives graphite composites its high tensile strength. Delaminations and simple matrix damage are less visible using eddy-current tests, but fiber damage is apparent with or without matrix damage.
Both eddy-current and ultrasonic techniques may be necessary for complete graphite-composite nondestructive testing, depending on the load-bearing requirements of the component. The load for components under tension is carried by fibers, and tensile strength is little affected by delaminations. Here, eddy-current testing would be an appropriate nondestructive testing tool. Loss of compressive strength in composites occurs when layers delaminate, a phenomenon to which ultrasound is particularly sensitive. For components which must survive both tension and compression, neither eddy-currents nor ultrasound alone provides an adequate indication of component serviceability, while a combination of the two techniques does.